Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Measurements of optical components and in particular the human eye have been addressed by a range of different instruments which have been able to provide information regarding different aspects of the eye's morphology and function as well as identification of various anomalies. Measuring the profile of the surface of the eye is of particular interest in applications where contact lenses need to be fitted, and as the range and functionality of contact lenses increases the requirement for accurate measurement of the surface topography over a larger area is becoming increasingly important. Other measurements that can be made include wavefront analysis, which is a phase-based measurement of the optical properties of the eye, i.e. eye function. Measurement of the various features of the anterior segment of the eye can be of great value in surgical applications. Considerable advances have been made in imaging the retina of the eye, and Optical Coherence Tomography (OCT) has enabled analysis of various eye structures in three dimensions through a scanning approach using information contained within the intensity and phase of reflected light.
FIG. 1 shows in schematic form a Placido disc topographer capable of calculating the elevation and curvature of a human cornea in vivo. A series of concentric illuminated rings 100 are reflected specularly from the target cornea 101, and an image of the reflections is projected by a lens system 102 onto an imaging sensor 103. Software is used to process the captured image to identify the ring reflections and the corresponding physical rings. Using the known geometry of the rings 100 and the lens system 102 a reverse ray trace 104 is performed between each ring image and the corresponding physical ring to determine the slope of the corneal surface at each reflection point, starting from the corneal apex 105. An ‘arc-step’ algorithm is used to calculate the slope, curvature and axial depth of the next ring reflection point along each radial.
FIG. 2 shows in schematic form a ‘push broom’ hyperspectral imager capable of spectrally analysing the intensity of light at a number of points along a slit aperture 222. A slit aperture 222 is used to analyse a linear portion of a sample 221, such as an image created by a telescopic or other imaging system. A collimating lens 223 directs the light collected by the slit aperture to a dispersive element such as a grating 225 which angularly disperses the wavelength components of the light, and a focusing lens 224 focuses each wavelength component onto a separate position along the wavelength axis of a focal plane array 226, where the information can be collected and analysed. A full hyperspectral image can be obtained by scanning the sample 221 relative to the slit aperture 222. Hyperspectral imaging has also been extended to two dimensional ‘single shot’ applications where light intensity at different wavelengths across an area is measured ‘single shot’ rather than in a scanning fashion.
Hyperspectral imaging only gathers intensity-related information, and any phase information is lost. It is of limited value to ocular metrology, and biological applications have been generally limited to understanding spectroscopic features such as oxygenation of blood, which is revealed in an absorption or fluorescence signature.
FIG. 3 shows in schematic form a wavefront analyser for determining the wavefront aberrations of an eye. An incoming known beam or wavefront 321, generally but not necessarily monochromatic, is transmitted through a beam splitter 322 to an eye under test 324 where the beam is focused by the eye's optical power 323, ideally onto or close to the retina 325. A small reflected component is then collimated by the eye's optical power and separated from the incoming beam 321 by the beam splitter 322 to form an outgoing wavefront 326, which contains information on residual optical power and aberrations of the eye 324.
The outgoing wavefront 326 is analysed with a Shack-Hartmann analyser 327 which, as shown in the enlargement in FIG. 3a, consists of a micro lens array 331 that samples the wavefront across a predetermined grid and focuses it onto a focal plane array 333. The positions of the image spots 334 formed by each micro lens can be used to estimate the slope 335 of the wavefront 326 at each sampling point, and if the slopes can be determined with sufficient accuracy and if the changes between the sampling points are not too great, it is possible to reconstruct the actual phase 328 of the wavefront across the sampling points. Multi-spectral wavefront analysers have been proposed for determining the dispersive properties of an optical component, e.g. longitudinal chromatic aberration of the eye, as discussed for example in P. Jain and J. Schwiegerling ‘RGB Shack-Hartmann wavefront sensor’, J. Modern Optics 55 (2008) 737-748, and in S. Manzanera et al ‘A wavelength tunable wavefront sensor for the human eye’, Optics Express 16 (2008) 7748-7755. However relative phase information between the different wavelengths is not obtained.
Many approaches to the analysis of the eye have relied on variations of a technique known as optical coherence tomography (OCT), which is able to provide tomographic data on eye structure and has been incorporated into many ocular instruments. There are two main approaches employed, time domain OCT and spectral domain OCT. In time domain OCT coherence properties of a partially coherent source such as a Superluminescent Light Emitting Diode (SLED) with a coherence length of several microns are utilised by imaging light reflected from a sample and interfering the image or a single point within the image with a reference beam provided by the same source, but with a time-varying path length. At a specific depth in the sample corresponding to the path length delay, an interference envelope of fringes will be detected in the combined back-reflected signal, allowing the reflection profile in the depth dimension to be reconstructed. Commonly this is done for only a single sample point at a time, and the corresponding scan of depth is known as an ‘A scan’. A variation of this technique, known as linear OCT, provides for the A scan to be captured in a single shot by appropriate angling of the reference and sample beams and detection of the fringes along a focal plane array. In each case the sample points can be scanned in an orthogonal dimension to provide a two-dimensional ‘B scan’ or even a complete three-dimensional scan.
Instead of scanning a delay line, spectral domain OCT techniques analyse the reflected light by interfering it with a reference beam, either as a time-varying function of wavelength (swept source OCT) or by dispersing the different wavelengths with a grating or other spectral demultiplexer and detecting them simultaneously along a detector array. The spectral domain information is the Fourier transform of the spatial (depth) reflection profile, so the spatial profile can be recovered (within the limitations of the technique) by a Fast Fourier Transform. Modern computational techniques enable fast A scans that can be scanned in two axes, with resonant scanning mirrors for example, to give complete high resolution scans at refresh rates which are a trade-off between the clinically permissible optical power, resolution and signal-to-noise requirements. It is known that in a scanning system utilising OCT it is difficult to achieve high accuracy relative measurements between the different sample points because of the micron level movements of the human eye in vivo that occur over period of a scan, which is typically of order one second.
Nguyen et al (Optics Express 21 (2013) 13758-13772) have proposed an OCT system based on combining an interferometer with a modified hyperspectral imaging system that is able to measure multiple A scans across an image plane. However because there is no calibration or method specified to guarantee phase relationships, this system appears to be unable to maintain relative phase information between sampling points or wavelengths.